The proposed work is to continue studies on the structure and genetics of the avian immune system with emphasis given to: (1) the structure of chicken allotypes; (2) the mechanism for expression of latent (unexpected) allotypes; (3) the fine structure of idiotypes as studied with monoclonal antibodies; (4) development of congenic chicken lines with hopes of mapping MHC genes; (5) the relation of immune responsiveness to related polymers to the MHC; (6) the cause of immunodeficiencies found in chickens.